I recently upgraded my router firmware to the OpenSource DD-WRT and consequently I now have a lot more information available to me than previously about my home network. One of the features that DD-WRT provides is a daily log of the amount of data the router uploads and downloads to the internet, and I'm gobsmacked to find out that over the fortnight or so since I've installed DD-WRT, I've apparently been uploading data at a rate varying between 5 and 15 Gb per day.
I can't think of any legit reason for this; eg although we have bittorrent here nobody's apparently been using it recently; nobody's been sending out massive emails or conducting huge file transfers to Dropbox or anything.
I run Avast antivirus and Malwarebytes antimalware (free versions) and neither has found anything untoward.
There are a lot of internet-linked devices and users here (computers, smartphones, iPad, TV, VirginMedia box etc) so difficult to pin down the culprit; that said, my desktop PC seems to spend an inordinate amount of time accessing its HDD, which I'd put down to Windows indexing...
I'm sure there must be a sinister explanation for this and Would be very grateful for advice from the experts as to where to go from here. Google ain't helping!